In present day copying machines or electronic printer devices, the fixing or permanent adherence of a toner image on a receptor material in the form of a sheet is mostly carried out with a so-called roller fixing system. This image-fixing unit comprises a pair of rollers through the nip of which a sheet carrying a toner image is fed. The surface of the roller contacting the toner image is heated above the melting temperature of the toner resin, which becomes tacky or molten and forms a permanent bond with the surface of the sheet. The roller, contacting the toner image, is provided with a coating showing abhesive properties versus the toner image. In order to increase the abhesive characteristics of the roller surface, use is often made of silicone oil. The rollers forming the image-fixing unit are pressed against each other with high force. The roller contacting the backside of the web is generally covered with a silicone elastomer, capable of resisting the heat generated by the image-fixing roller.
Problems arise with hot roller image-fixing devices. In particular, in heavy duty printers where long periods between servicing are usual, it is difficult to maintain a constant image-fixing quality.
Where the receptor material is in the form of a continuous web, there is a need to synchronise the drive to the image-fixing rollers to the movement of the receptor material therethrough, but this may be difficult where the surface of one or both of the rollers is resilient, leading to uncertainty in its circumferential speed. There is a need to accurately align the image-fixing rollers, otherwise damaging skewing forces on the web may result. Roller image-fixing systems, due to the high pressure exerted in the nip formed by the rollers, also can be the cause of the toner image being smeared, especially in colour work. A second effect is that the high pressure gives rise to a fixed image showing an increased, and often unwanted, gloss. This degrading or "calendering" effect may even be transmitted to the receptor material itself so that its surface structure may wholly or partially be lost. Furthermore, damage to the receptor material, and to the image thereon, may occur if the receptor material is in contact with the heated roller for too long a period of time, such as if the movement of the receptor material were to stop for whatever reason.
Furthermore, hot roller fixing is not easily compatible with double-sided (ie, duplex) printing.
A technique known as "flash-fixing" is also known in which a short intense burst of radiant energy is applied to the receptor material carrying the toner image to be fixed. The wavelength of the radiant energy is chosen to be absorbed by the toner and is therefore in the visible or ultra-violet part of the spectrum. Such a technique is unsuitable for multi-colour images, where toners of different composition are carried on the receptor material, said toners having different adsorption characteristics in the visible spectrum.
A number of constructions of image fixing stations have been proposed in the art. Thus, British patent GB 1590872 (Xerox Corporation) describes an electrostatographic printer in which, after the transfer of toner images to a paper web, the images are fixed by radiant infra-red heaters. U.S. patent U.S. Pat. No. 3,449,546 (Dhoble/Xerox Corporation) describes a xerographic fusing apparatus, which is capable of heating toner powder to its melting point without damaging the paper support material, wherein the paper acts as a heat source to aid in the fusing process. German Offenlegungsschrift DE-A-2506953 (Itek Corp.) describes a fusing apparatus in the form of a hinged two-part housing, having open and closed positions, one part containing an active heat source and the other part containing a passive heat source which is heated by the active heat source in the closed position. This arrangement has the disadvantage that once the housing is in its open position, the indirect heat source loses-its heat, with the result that the heat output of the apparatus as a whole is not constant.
Where the fusing apparatus is used to fix toner on a moving web, especially when that web is formed of paper, a dangerous situation can arise in the event of a failure of the web drive, such as may occur if there is an electrical power failure. Even if electrical power to the fusing apparatus is cut at this time, a portion of the web stands in close proximity to the fusing apparatus which may retain significant heat for some time. There is therefore a risk that the web material will be damaged or may even catch fire. To the best of our knowledge, the prior art has not addressed this particular problem.
It is an object of the invention to provide an electrostatographic printer which avoids one or more of the aforementioned problems.